Steam sterilization method



Jan. 13, 1959 E. J. PorrRAs` STEAM sTERILIzATIoN METHOD 2 Sheets-Sheet 1 Filed April 14, 1954 I I I I I I I I I I I I I I I I I I I I I Edwuvd elPaidfm,

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' STEAM STERILIZATION METHOD Filed April 14, 1954 '2 sheetssheet 2 s Je y Iszveaaod;

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t @fi Zlb Patented `lain. i3, 1959 fine 2,868,616 `STEAM sTERILrzArIoN METHOD "Edward .i Poitras, Holliston, Mass. Application April 14, 1Q54, Serial No. 423,185

sclaims. `(ci. 21a-s6) The present invention relates generally to sterilization and more particularly to a method and apparatus for autoclaving or sterilizing closed or sealed containers charactei-ized lby limited-strength or vapor-permeable walls and having a fluid content or charge. Such containers are exemplified by bags, pouches or sheaths associated with` the storing unit of a recently introduced flexible, collapsible blood handling equipment, which bags, pouches or sheaths may be provided or charged in manufacture with a determined quantity of fluid; meaning by fluid--liquid, gas, or liquid-gas mixture. The invention aims to provide for the sterilization of such containers by means and method avoiding and eliminating injury to the same and preventing also loss of the liquid, gas or liquid-gas container atmosphere or charge.

The invention will be better understood from a consideration of the following specificationtaken in conjunction with the accompanying drawing in which:

Figi is a partly schematic representation of a sterilizing chamber as novelly fitted for automatic operation in accordance with the invention; and

Fig. 2 illustrates the application of the invention method to a` conventionalautoclave.`

Effective sterilization by steam is conventionally understoodto require complete exposure of the article to the steam at or above predetermined temperatures and pressuresand continuously for a minimum period of time. A technically acceptable method may be, forexample, exposure to steam at 250 F. for a period of at least one-half hour. To insure the required complete, total exposure to steam,` upon loading or charging of the autoclave with the articles to be sterilized, the sterilizing chamber gases must in the conventional practice be expelled, as by evacuation, or by layeringf and the contents sealed off from exposure to air and the like atmospheric gases. The presence of air, along with retarding the rate of heat transfer to the material to be sterilized, reduces the temperature and prevents sterilization, in that the temperature is generally found from the total pressure, which of course is increased by the presence of air or other gases in the autoclave.

The conventional steam sterilizing method as above described is inapplicable to apparatus incorporating closed or sealed `chambers or containers subject to explosion or implosion, when exposed to temperature change, by reason of its manufacture 'from either limitedstrength or vapor-permeable material. Certain recently introduced Y blood handling and administering equipment, such as disclo-sed in Walter Patent No. 2,702,034, is fashioned of a vapor-permeable plastic, therein indicated as polyvinyl chloride or polyethylene and distinguished also by bags, pouches and sheaths whichhave 'certain volume limits and a predetermined liquid, gas, or liquid-gas atmosphere or charge. lt will be appreciated that in the sterilization of this equipment, as by the method abo-ve noted, charge volume increase ordecrease sufficient to burst or collapse l the containers and also loss through diffusion of the con- Similarly, `aclosed, flexible plastic pouch containing only gas, as air, would be subject to undesiredsticking together and possible permanent deformation of 1ts parts upon its collapse, flowing from the pressure disparlty set up 1nitially by the introduction of the steam in conformity to the yconventional method. Moreover, the gradual diffusion of the steam through the pouch walls would create a steam pressure combining with the air pressure to develop a total steam and air pressure within the pouch exceeding the total steam pressure without the pouch and bursting the pouch before sterilization is completed. Finally, a closed sealed bag, pouch, sheath or other container of such flexible plastic `construction and having a 1quid-gas or air-water charge would burst upon the introduction of `the steam in the conventional sterilizing practice by reason of the difference :between the internal or container pressure, being the `sum of the partial gas pressure of the heated air and the partial vapor pressure of the heated water, and the external or sterilizer pressure, being merely the pressure of the sterilizing steam thus introduced. j l l l The present invention provides novel method and means whereby the sterilization of limited-strength and vaporpermeable chambers or containers such as above mentioned may be safely and effectively carried out. More particularly, the air and steam pressures within the sterilizing vessel o-r autoclave are controlled and manipulated under the invention to conform to or counterlbalance the internal pressures developed in the containers during the sterilizing cycle and thus to prevent the described bursting or collapse of said containers and inhibit the diminution of their atmospheres.

ln accordance with the invention, in the sterilizing of a limited-strength or vapor-permeable container, total pressures are maintained in the autoclave at least equal to the total pressures built up or obtaining within the container throughout the sterilizing cycle. It will be understood that a slight and momentary external pressure excess such as may be necessary to actuate control devices will safely and quickly be nullified by limited and harmless container compression or contraction. Thus the construction and operation of an autoclave in accordance with the invention method requires the provision and control of fittings adapted to introduce steam and airl or other gas at varying `control pressures, to permit the venting of excess steam or air, and to maintain a sterilizing temperature for a fixed period of time.

Referring now more particularly to the drawing, there is shown schematically in Fig. l a steam chamber 10 having a drain or trap lll and a fan l2. Steam is admitted to the chamber lil through line )i3 having valve 14 which may be controlled for automatic operation by a temperature sensing element l5. Air is admitted to the chamber lit) through line lr6 and valve Il?, which is automatically operated as hereinafter described. Escape for the chamber gases is provided by the vent i8 as limited or permitted by automatic control valve 19.

ln accordance with the invention, means are provided for automatically controlling the steam and air supply to and the venting of the autoclave so as to effect or provide v total steam chamber pressures counterbalancing the total pressures developed within the limited-strength and/or` vapor-permeable arti-cles to be sterilized. The desired control and adjustment of the chamber fittings fo-r continuous and exact pressure counterbalance would require,

ideally, the use of one of the articles themselves as the container pressure detecting means. I have found, however that a sufficiently accurate and a more practical co-ntrol instrument is provided by a variable volume or bel-v lows element or device such as that indicated at 2t), Fig. l, and there shown to be supported from a wall of the steam chamber as by a hanger plate 21 and mounting screw 22. The bellows 20 is seen to control a leverarm 23, mounting a contact 24 for reciprocating movement through the central position shown and oppositely into engagement with the fixed contacts 25, 26. The one contact 2S is connected as shown for controlling solenoid-actuated or self-operated air supply valve 17, and the other contact 26 is connected for controlling the similarly operated chamber vent valve 19.

In accordance with the invention, the chamber pressure control device or bellows 20 is arranged and conditioned for simulating the thermal characteristics of, and more particularly for reproducing the pressure changes in, the limited-strength or vapor-permeable containers or bags with which the invention is concerned. To this end the bellows 20 is provided with a fill port, herein the screw 27 and gasket 28, through which it is charged with gas and liquid of the type and in the quantity calculated to generate therein partial pressures duplicating those developed in the bags.

While the bellows 20 is conveniently of metallic construction, it will be understood that it may be desirable to simulate also the bag permeability characteristic. Accordingly, and for sterilizing blood bags or other sealed containers characterized by vapor-permeable walls, the bellows unit may be fashioned in whole or in part of the same or similarly permeable material as the containers.

The manipulation of an autoclave fitted like that just described and for operation in accordance with the invention method is relatively simple. Considering now the example of a container or containers having a sealed liquid-gas or air-water content or charge, the sterilizing cycle is initiated by setting or adjusting the steam valve 14 for automatic control by temperature sensing element 15, and then switching on the fan 12. Any convenient line may be tapped for the steam, so long as it can supply a pressure at least as great as that required Vto balance the total pressure calculated to develop within the containers. The steam is thus admitted to the sterilizmg chamber at or above a predetermined or selected pressure and in volume to substantially fill the chamber and to raise its temperature to the desired level.

My novel method is importantly characterized in this chamber-filling stage and subsequently by the elimination of steam and air layers and the thorough dispersal of the steam throughout the autoclave chamber. This will be understood to be eitected by the steam-air mixing fan 12, which thoroughly agitates the steam and urges it into complete sterilizing Contact with the chamber contents.

As the introduction of the steam proceeds, the autoclave chamber temperature will mount until it has reached the control point, herein 250 F., when the steam supply is automatically shut ott, the temperature sensing control 15 being set to close steam valve 14 at that level.

This raising of the chamber temperature to well above the atmospheric boiling point will be understood to raise the internal vapor and gas pressures of the containers and of the bellows to a total pressure at which containers such as here concerned are subject variously to bursting and undesired vapor or gas transfer through permeable walls, in the absence of a counterbalancing pressure external to or without the same.

y In accordance with the invention, the pressure changes in the containers are duplicated by pressure changes in the control instrument, and such pressure changes in, and resultant movement of, the bellows 20 will drive or shift the lever arm 23 and contact 24 from the intermediate Fig. l position into engagement with one or the other of the fixed contacts 25, 26. The initial expansion or leftward movement of the bellows will be seen to rock the lever arm downwardly into engagement with contact 25, controlling valve 17 to open line 16 and admit compressed air, or other gas. The air, like the steam, is supplied at a pressure at least as great as that necessary to counterbalance the total container pressure.

Further in accordance with the invention, the control .to vent the autoclave.

A device or bellows 20 functions to admit this pressure air until a total internal-` or container-pressure counterbalancing pressure is built up in the autoclave. This container-pressure counterbalancing pressure may be defined as a pressure equal tothe sum of the partial steam or other vapor pressure and thepartial air or other gas pressure'obtaining in the container under sterilizing conditions. VThe total chamber pressure is thus controlled by manipulation of the air supply, which is automatically shut off when the desired equilibrium is reached, the bellows 20 and lever arm 23, 24 then returning to the initial or intermediate Fig. l position...

K In the event of an external or chamber-pressure excess, ,the control bellows will be collapsed or driven oppositely or to the right thro-ugh its Fig. 1 position, and so as to pivot the lever arm 23 and contact24 into engagement with fixed contact 26. This energizes or opens valve 19 When the chamber pressure excess is thus dissipated, the bellows will expand and return the lever'arm 23 once again to the intermediate Fig. l position. x

It should be noted that the artificially produced chamber pressure may be manipulated variously to exceedror be less than, by any desired amount, a particular total container pressure. of a plurality of containers of differing iluid content, as

a bag charged with a liquid and a pouch filled with a gas, which bag and pouch are seen to heat up or cool down at different rates and thus to develop different 1nternal pressures Vduring'at leastl part'of the sterilizing cycle, a controller, normally functioning to provide a vwhen full automatic control of the chamber pressure is not required, 'or when an approximation or imbalancel of chamber and container pressure is sulicient or useful. It is possible in any particular case to predict and reproduce artifically the approximate container pressure conditions characterizing the several stages of a sterilizing cycle.

In Fig. 2 there is shown a conventional autoclave as fitted for operation in accordance with the invention method, and more particularly for automatic operation by control means providing an approximation of kcontainer pressure conditions. The s'team chamber proper is therein indicated generally at 30 and mounts a valved drain 31 and a fan 32. A steam supply conduit 33 is supported through a chamber wall and carries an automatic steam valve 34 which is connected to and controlled by a temperature sensing element 35 disposed within the steam chamber, as shown. The supply pressure may be regulatedvby a reducing valve 36 arranged outwardly of the steam valve 34 on the pipe 33.

The autoclave is apertured near its top wall by a vent pipe 37 which carries an automatic valve 38, the latter connected to and controlled by the pressure sensing element 39. Air or other gas is supplied to the steam cham ber through a conduit 40 which is seen to be branched, as at 40a, 4012. The one branch 40a carries a manual air valve 41. The other branch 40b mounts in series automatic or pressure kvalve 42 and outwardly thereof automatic air valve 43, the latter controlled-by a chambermounted temperature sensing element 44.

The manipulation of the controls and fittings of the autoclave of Fig. 2 for sterilizing operation in accordance with the invention method is predicated upon an arbitrary calculation or approximation of the pressures generated within the limited-strength or vapor-permeable `container. In the case ofthe herein `above-mentioned Thus in controlling the sterilization Partial and total container pressures (p. s. z'. a.) at ambient and sterilizing temperatures (F.)

Ambient Sterilizing Temp. Temp. (80) (250) Moisture vapor pressure 0. 5 29. 8 An' pressure 14. 7 19. 3

Total container pressure 15. 2 49. 1

The table shows in the right-hand column that the autoclave chamber steam and air pressures should, in the instant example, be controlled at approximately 30 and 19.3 p. s. i. a. respectively for and at a sterilizing temperature of 250 F.

The sterilizing cycle is initiated, similarly as with the Fig. 1 apparatus, by turning the steam valve 34 to the automatic position, and switching on the fan 32. Referring to the table, and assuming an atmosphere of 14.7 p. s. i., it will be seen that the steam pressure reducing valve 36 and also the air supply pressure regulating valve 42 should iirst be set to supply the steam and air respectively at or slightly in excess of 49.1 p. s. i. a.

The initial setting of the autoclave controls as just described will serve to charge the sterilizing chamber with steam until the temperature has reached the desired level, herein 250 F., at which the temperature sensing control 35 may be set to close automatically the steam supply valve 34. It will be seen from the graph that during this chamber temperature increase and steam pressure build-up, the container air pressure is calculated to increase to a value of 19.3 p. s. i. a. In accordance with the invention this is counterbalanced by setting the air supply control element 44 to open the air valve 43 at, say 245 F., so that over the last 5 of temperature rise, air can be introduced into the chamber along with the steam in the event that the original charge of air has been reduced through leakage.

Further in accordance with the invention, the pressure sensing vent control 39 is set to open the valve 38 automatically should the chamber pressure exceed the indicated control Value, and to close the chamber Vent when the pressure excess has been dissipated. Thus it will be seen that any departure from desired sterilizing temperature and counterbalancing pressure conditions in the autoclave are immediately and automatically corrected.

The pressure balancing technique of the invention is importantly brought into play also during the cool-down of the autoclave prior to removal of the sterilized articles. In accordance with the invention, in this cooldown or take-down the steam valve 34 is manually closed, and the air supply valve 43 held open until the autoclave load has cooled to well below the atmospheric boiling point, say 175 F., when the air in the sterilizer may safely be relieved to atmospheric pressure, as by opening the vent valve 38. Thus the autoclave is rapidly cooled by the continuous introduction of pressure air, which blows out the steam while maintaining desired pressure counterbalance.

It will be understood that the operation of a sterilizer in accordance with the invention, and whether by automatic or manual means, may vary in the case of the vaporpermeable container according as it is manufactured with a liquid, gas, or liquid-gas charge. Thus in the case of a collapsible, vapor-permeable, partially air-filled container the initial introduction of the sterilizating steam into the autoclave will, as earlier noted, produce a pressure disparity likely to collapse or implode the container and to cause undesired sticking together or deformation of its walls. This is prevented in accordance with the invention by an initial venting of the autoclave air. ln the course of the sterilizing cycle there occurs a vapor permation through the container walls, creating incremental pressure, or internal pressure increase, tending to burst or explode the containers. This is prevented by subsequent replenishing of the autoclave air, such as to re-establish a counterbalancing total autoclave pressure.

From the foregoing it will be appreciated that by the novel means and method of the invention a sterilizing vessel is provided with a desired mixture of vapor and gas, as steam and air, at controlled pressures substantially equal to those generated in and during sterilization of containers of the class having a gas, liquid, or liquid gas charge and vapor-permeable or limited-strength walls. In other words, an autoclave is operated in the sterilizing cycle with the chamber pressure automatically regulated at a level counterbalancing that calculated to develop 0r measured in the containers undergoing sterilization. Thus there is maintained an equilibrium pressure condition preventing the undesired bursting or collapse of and loss of the liquid, gas, or liquid-gas charge from limitedstrength and vapor-permeable containers such as here concerned. The invention method is distinguished by the controlled introduction of a gas into the sterilizer, and by the agitation or mixing of the sterilizer air-steam mixture during at least a substantial portion of the sterilizing process for complete exposure of the articles undergoing sterilization. The invention is characterized further by control of both temperature and pressure, the temperature by regulation of the steam supply, and the total pressure by regulation of the air supply.

It will be understood that' my invention, either as to means or method, is not limited to the exemplary embodiments or steps herein illustrated or described, and l set forth its scope in my following claims.

I claim:

l. The method of sterilizing closed collapsible vapor permeable containers charged with liquid, gas and liquidgas mixture which comprises placing the containers in an autoclave having a vent, supplying steam and gas to the autoclave agitating the autoclave atmosphere during at least a substantial portion of the sterilizing process for complete steam-gas mixing and for total steam-gas contact with the containers, regulating the autoclave steam supply to establish a sterilizing temperature for a sterilizing time, and regulating the autoclave gas supply and the autoclave vent to establish a substantial balance across the walls of the containers of the partial pressure of the gas in the autoclave with the partial pressure of the gas in the containers.

2. The container sterilizing method of claim l, wherein the gas supplied to the autoclave is selected to match that in the containers.

3. The container sterilizing method of claim 1, and closing off the autoclave steam supply, continuing the autoclave gas supply to blow out the autoclave steam, and continuing the regulating of the autoclave vent to maintain during said blowing out and until the autoclave and containers are suiiiciently cooled an autoclave pressure counterbalancing the sum of the partial container pressures.

References Cited in the tile of this patent UNITED STATES PATENTS 1,207,814 Stockton Dec. 12, 1916 1,385,609 Durkee July 26, 1921 1,485,133 White Feb. 26, 1924 2,472,970 Hanna June 14, 1949 2,539,505 Barnum Ian. 30, 1951 2,555,230 Ford May 29, 1951 2,578,437 Martin Dec. 11, 1951 

1. THE METHOD OF STERILIZING CLOSED COLLAPSIBLE VAPOR PERMEABLE CONTAINERS CHARGED WITH LIQUID, GAS AND LIQUIDGAS MIXTURE WHICH COMPRISES PLACING THE CONTAINERS IN AN AUTOCLAVE HAVING A VENT, SUPPLYING STEAM AND GAS TO THE AUTOCLAVE AGITATING THE AUTOCLAVE ATMOSPHERE DURING AT LEAST A SUBSTANTIAL PORTION OF THE STERILIZING PROCESS FOR COMPLETE STEAM-GAS MIXING AND FOR TOTAL STEAM-GAS CONTACT WITH THE CONTAINERS, REGULATING THE AUTOCLAVE STEAM SUPPLY TO ESTABLISH A STERILIZING TEMPERATURE FOR A STERILIZING TIME, AND REGULATING THE AUTOCLAVE GAS SUPPLY AND THE AUTOCLAVE VENT TO ESTABLISH A SUBSTANTIAL BALANCE ACROSS THE WALLS OF THE CONTAINERS OF THE PARTIAL PRESSURE OF THE GAS IN THE AUTOCLAVE WITH THE PARTIAL PRESSURE OF THE GAS IN THE CONTAINERS. 